1. For the Sri Lankan curry power,lightly toast the whole spices and grind to a fine powder,then add the turmeric.
2. For the coconut pol sambol,soak the desiccated coconut in the coconut milk and leave for 30 minutes. Finely grind the peppercorns and Maldive fish flakes to a fine powder using a mortar and pestle. Mix ingredients together,adding lime juice a little at a time until flavour is balanced.
3. For the Sri Lankan curry sauce,place the oil into a heavy-based saucepan and add the onions and sea salt and cook at medium heat for about 5 minutes until softened,then add the garlic and chillies and cook for a further 5 minutes. Next add the lemongrass and fish curry powder and cook for another 5 minutes. Add the curry leaves,tamarind purée and coconut cream,simmer for 2 minutes.
4. Add prawns and snake beans. Bring to just below the boil,allow to heat to gently cook the prawns,about 2-3 minutes,stirring often. Add a tablespoon or two of water if needed. Remove from heat,cover and stand for 5 minutes,then season to taste and add the remaining lime juice if needed,plus extra curry sprigs if using.
5. Top curry with sambol and serve with rice pilaf(recipe here).